Optical proximity sensors are active sensors that are used to detect the presence or distance of an object within their measuring range. Typically an optical proximity sensor comprises a light source to emit light towards an object, and an optical sensor that detects the amount of light which eventually gets reflected back from the object. The light source generates visual, infrared or ultraviolet (UV) light depending on the spectral sensitivity of sensor. Typically, the light source is chosen to emit light that is not likely to be generated by other nearby sources. Infrared emission is used in common optical proximity sensors. The sensor typically is a semiconductor device such as a photodiode, which generates a current when light energy strikes it.
Optical crosstalk has always been the most significant problem for optical proximity sensors, due mainly to Fresnel reflections off of the inside and outside surfaces of glass or plastic covers that typically cover the proximity sensor in cell phone and tablet products, for example. Fresnel reflection or specular reflection denotes mirror-like reflection of light, or of other kinds of electromagnetic waves, from a surface, in which light from a single incoming direction is reflected into a single outgoing direction, determined by the law of reflection. While there are other non-specular ray paths (generally caused by random scatter), the specular reflection is usually the biggest contributor.
Attempts are made to control optical crosstalk by using small air gaps between sensor and cover, rubber boots, specially inked apertures on the phone glass, and specially shaped apertures on the shields. Even with dedicated system design, the crosstalk can still be problematic in a final product due to variations in factors such as air gap, lateral device alignment with respect to apertures and the like.